1. Field of the Invention
The present invention relates to a push-on switch having a long operation stroke, which is utilized, for example, in a keyboard for an electronic device.
2. Description of the Prior Art
Recently, following size reduction and functional advancement of electronic devices, there has been an increased demand for push-on switches having long operation strokes for avoiding misoperation, in addition to having reduced size and thickness and providing enhanced feedback operation feelings.
Japanese Unexamined Patent Publication No. 3-214519 discloses a long-stroke push-on switch, which will be explained with reference to FIGS. 8 and 9A-9B.
FIG. 8 is a front-sectional view of the conventional long-stroke push-on switch. In the figure, numeral 1 denotes a box-type switch case made of resin by molding. At the bottom of the switch case 1, a pair of outer fixed contacts 2 and a central fixed contact 3 are fixed through insert molding. The outer fixed contacts 2 are connected to a connecting terminal 2A while the central fixed contact 3 is connected to a connecting terminal 3A. A movable contact member 4 in the form of an elastic metal thin plate is disposed on the outer fixed contacts 2.
FIGS. 9A and 9B are a plan and a front view of the movable contact member 4, respectively. As shown in these figures, the movable contact member 4 comprises a tongue contact portion 4A formed at the center by blanking the metal thin plate, and a peripheral portion 4C continuous with the tongue contact portion 4A at a root portion 4B. The tongue contact portion 4A is bent at the root portion 4B so as to protrude upward. The peripheral portion 4C is disposed on the outer fixed contacts 2 and thus electrically connected thereto, while the central tongue contact portion 4A confronts the central fixed contact 3 with a gap therebetween in the non-actuated state of the switch.
Numeral 6 denotes a dome-shaped spring member made of an elastic material, such as rubber. The spring member 6 has a dome-shaped hollow body 6A whose lower end is disposed on the peripheral portion 4C of the movable contact member 4. The spring member 6 further comprises an upward projection 6B and a downward projection 6C provided at the top of the dome-shaped body 6A. A push button 7 is placed on the upward projection 6B, while the downward projection 6C confronts the tongue contact portion 4A of the movable contact member 4.
The push button 7 comprises a lower portion 7A having a pair of projections 7B formed on the outer periphery thereof, and an upper portion 7C continuous with the lower portion 7A. The lower portion 7A is received in the switch case 1 such that the outer periphery of the lower portion 7A is vertically slidable along the inner periphery of a side wall 1A of the switch case 1 without inclination. Further, the projections 7B are slidably received in a pair of vertical grooves 1B formed on the inner periphery of the side wall 1A so as to be prevented from rotation and slipping off upward.
Now, an operation of the conventional push-on switch will be explained. When the top surface of the push button 7 is pressed by an operation button (not shown) of a using device, the dome-shaped spring member 6 in the switch case 1 is pushed downward by the push button 7 so as to be elastically deformed with tactile feedback to a user's finger. Then, the downward projection 6C of the spring member 6 pushes downward the tongue contact portion 4A of the movable contact member 4 so that the underside of the tongue contact portion 4A abuts the central fixed contact 3. Thus, electrical connection is achieved between the outer fixed contacts 2 and the central fixed contact 3, that is, between the connecting terminals 2A and 3A. Thereafter, when the push button 7 is released, the switch returns to the state shown in FIG. 8 due to elastic restoring forces of the dome-shaped spring member 6 and the movable contact member 4 so that the connecting terminals 2A and 3A are disconnected from each other.
As described above, in the foregoing conventional push-on switch, the dome-shaped spring member 6 in the switch case 1 is pushed downward by the push button 7 so as to be elastically deformed with the tactile feedback to the user's finger. However, for holding the push button 7 in the switch case 1 so as to be vertically movable along the inner periphery of the side wall 1A without inclination, a length H1 of the lower portion 7A of the push button 7 is required to be considerably large. Further, for reliably operating the switch, a length H2 of the upper portion 7C projected from the switch case 1 is required to be larger than an operation stroke of the switch. Accordingly, it is difficult to reduce the height of the switch.
Further, it is difficult to form, with high accuracy, the vertical grooves 1B on the inner periphery of the side wall 1A of the switch case 1 by molding with resin, particularly, in the small-size switch.